Fast beam of neutral atoms created using lasers and plasma

Computer simulation of a laser plasma accelerator, showing the waves in the plasma that accelerate charged particles. A new experiment has extended this design to accommodate neutral particles as well.

Electrically charged particles are relatively easy to accelerate using electric and magnetic fields. Neutral particles cannot be steered in the same way, which is a bit disappointing, since they are useful experimentally due to their much greater ability to pierce target materials. A promising technique uses very short pulses of intense laser light to accelerate neutral particles, though up until now it has only achieved low energy transfer.

A new experiment has achieved neutral particle energies on the order of a billion times greater than prior efforts. R. Rajeev and colleagues managed this by accelerating particles while they were charged, and then transferring in electrons to neutralize the charge. This method has the advantage of being compact, and therefore useful for applications such as nanolithography; however, the authors argued their approach can be generalized for other purposes.

Laser acceleration works by bombarding atoms with light pulses to strip electrons off, creating a plasma—a neutral gas of ions and electrons. Since the electrons are a lot less massive than the ions, you can tune the laser pulses in a precise way that separates them. The electrons shoot off, leaving behind a gas of positively charged atoms moving in coherent waves.

As we all know from elementary school physics, like charges repel. So any positively charged particle added to the plasma will experience acceleration from the plasma waves. Laser plasma accelerators are more compact than many other accelerator designs, including those used in big experiments like the Large Hadron Collider (LHC), although they haven't yet reached the same energies. Plasma acceleration (sans lasers) is also important in many astrophysical processes.

To adapt the method to neutral particles, the authors of the new study began with all the steps in the previous paragraph, using a plasma of argon atoms. However, the free electrons in the plasma were recombined with non-accelerated ions to create Rydberg atoms, in which electrons are only barely attached to the nucleus. Those loosely connected electrons were then transferred via collision to the rapidly moving ions from the accelerator, neutralizing them.

After applying powerful electric fields to separate out any residual ions, the researchers were left with a directed beam of highly energetic neutral atoms. By piggybacking on laser plasma acceleration, they achieved megaelectronvolt (MeV) energies, a billion times greater than previous efforts.

For comparison, these are equivalent to the energy of a large van de Graaf generator like that on display at the Boston Museum of Science. The LHC achieves energies a million times greater still—teraelectronvolt (TeV)—using magnetic fields. But, unlike these other experiments, the laser plasma accelerator was desktop-sized.

Neutral particles have greater penetrating power when striking a target for the same reason they are hard to accelerate: they are not affected by electromagnetic fields. That makes them useful in nanolithography: "printing" surfaces with patterns that have specific electronic properties. The researchers also proposed extending the technique to lighter atoms, specifically hydrogen. This could lead to advances in plasma physics, as well as application to more conventional particle colliders, providing new avenues for research.

This also has the advantage of not creating a charge buildup on the surface that is bombarded. Ion beams have unpredictable effects on a surface if used for too long without charge compensation. I was hoping this could be used as a new form of mass spectrometry (the technique that allows us to measure the masses of atoms and molecules). But with requirement to form ions first, there is likely no advantage over existing mass spec designs. Still a very cool development.

Little bit confused. Having accelerated ions slam into the Rydberg atoms would slow them down. Were the MeV energies reported for neutral atoms the speed after this step? What energies were the ions at before they interacted with the Rydberg atoms?

This also has the advantage of not creating a charge buildup on the surface that is bombarded. Ion beams have unpredictable effects on a surface if used for too long without charge compensation. I was hoping this could be used as a new form of mass spectrometry (the technique that allows us to measure the masses of atoms and molecules). But with requirement to form ions first, there is likely no advantage over existing mass spec designs. Still a very cool development.

But what if we use the neutron beam to knock electron out to make ions? Same as FAB but with neutron.

Or create a plasma using laser (skip the Rydberg electron) instead of RF (i.e. ICP) I wounder if it would work with condensed phase (not gas)

I'm a little confused, because neutral beams in general are not novel. How does this technique differ from neutral beam heaters used in tokamak test reactors? Is this a breakthrough because of the high energy of the individual particles, or the using a laser to start the process, or something else?

Anybody know whether this would make an ion drive variant? - sounds ideal if the effiency is reasonable. Plus, of course, Dan Dare or Flash Gordon could adapt it to take out passing North Korean or Al Quaeda satellites.

Would this be suitable for spacecraft electric propulsion? Does it beat the 72% power efficiency of the VASIMR?

The article mentions it's a desktop-sized installation, so I'm guessing the weight factor would compensate for the lower efficiency.

resthavener wrote:

Anybody know whether this would make an ion drive variant? - sounds ideal if the effiency is reasonable. Plus, of course, Dan Dare or Flash Gordon could adapt it to take out passing North Korean or Al Quaeda satellites.

It doesn't seem like this setup would have any benefit to accelerators used for propulsion. As the article states, neutral beams are primarily desired because they are much harder to divert from their course, giving them a better penetration.

For propulsion you don't particularly care about what the particles--charged or not--do once they leave the engine, you only care about how much mass is leaving your engine how fast, and how much energy it took to accomplish that.

Yonder is correct. A neutral beam provides no benefit over a charged one for the purposes of engine acceleration. On the other hand, if you're using it as a weapon against a magnetically shielded target, this will be a big deal

Anybody know whether this would make an ion drive variant? - sounds ideal if the effiency is reasonable. Plus, of course, Dan Dare or Flash Gordon could adapt it to take out passing North Korean or Al Quaeda satellites.

A bit of Googling tells me that a flying mosquito has about 1 TeV of energy - so each atom accelerated this way has about 1/1000000 the impacting power of a flying mosquito crashing in to you.

I was disappointed at first but I have no way to determine how many atoms you could accelerate this way and how well they would be focused - mole quantities all impacting the same tiny surface area at high velocity should still have destructive power.

How well you could maintain the beam through air is certainly a problem for terrestrial use.

Oh man. One step closer to the beam weapons and particle weapons from my sci-fi dreams. Now they just have to discover the minovsky particle.

I seriously doubt that such things will ever beat even current weapons tech. A thermo nuke sounds a lot scarier to me than any ray gun idea I've heard.

Is it really fair to compare a line of sight weapon to a high yield area of affect weapon? A particle beam wouldn't replace a nuclear bomb, that's not it's competition (it will leave that to the antimatter bomb). A particle beam would replace another line of sight weapon, like a rifle.

You can still make the argument that new fangled energy weapons will never beat good old "metal propelled by oxidation reaction" standby, but make that argument, not some arbitrary one.

Unfortunately, since the free abstract of the article doesn't mention how many of these megaelectronvolt-argon atoms was in every "shot" of the gun, I can't make an apples to apples comparison of the amount of energy in a "shot" of the new particle accelerator as opposed to something more pedestrian like an M16.

However, since this last accelerator just boasted an improvement of 9 orders of magnitude, and we are pretty much at the limit of "metal propelled by oxidation reaction" I wouldn't be surprised to see our current type of weapons obsoleted soon. The question is what will replace them, lasers, particle accelerators, or rail guns like the Navy is already working on?

The LHC achieves energies a million times greater still—teraelectronvolt (TeV)—using magnetic fields.

The LHC does not use magnets to accelerate the beam: the magnets simply bend the beam into a circle. High-quality microwave cavities contain the strong electric fields responsible for the acceleration.

Pfft, sucks compared to *my* neutral particle accelerator. You've just got to think out of the box. I just put a nice little black hole behind the target, and voila, the particles slam into the target real good. Bit difficult preventing myself, the target, and the accelerator from also being accelerated, but hey, that's what engineers are for.

Pfft, sucks compared to *my* neutral particle accelerator. You've just got to think out of the box. I just put a nice little black hole behind the target, and voila, the particles slam into the target real good. Bit difficult preventing myself, the target, and the accelerator from also being accelerated, but hey, that's what engineers are for.

Easy to keep the target from accelerating into the black hole. Just mount it to the black hole on a long pole. Problem solved.

Pfft, sucks compared to *my* neutral particle accelerator. You've just got to think out of the box. I just put a nice little black hole behind the target, and voila, the particles slam into the target real good. Bit difficult preventing myself, the target, and the accelerator from also being accelerated, but hey, that's what engineers are for.

Easy to keep the target from accelerating into the black hole. Just mount it to the black hole on a long pole. Problem solved.

Pfft amateurs. You just need ANOTHER blackhole equidistant from the target blackhole.

This also has the advantage of not creating a charge buildup on the surface that is bombarded. Ion beams have unpredictable effects on a surface if used for too long without charge compensation. I was hoping this could be used as a new form of mass spectrometry (the technique that allows us to measure the masses of atoms and molecules). But with requirement to form ions first, there is likely no advantage over existing mass spec designs. Still a very cool development.

Pfft, sucks compared to *my* neutral particle accelerator. You've just got to think out of the box. I just put a nice little black hole behind the target, and voila, the particles slam into the target real good. Bit difficult preventing myself, the target, and the accelerator from also being accelerated, but hey, that's what engineers are for.

Easy to keep the target from accelerating into the black hole. Just mount it to the black hole on a long pole. Problem solved.

Pfft amateurs. You just need ANOTHER blackhole equidistant from the target blackhole.